The invention disclosed in U.S. Ser. No. 07/639,568 relates to a particulate filter, regenerable by back-flushing, formed from a porous honeycomb monolith structure with selectively plugged passageways and a microporous membrane coating applied to the passageway surfaces. This continuation-in-part application further includes a catalyst coating within or on the filter, which becomes a catalytic filter, capable of reacting constituents in the filtrate fluid as it passes through the filter.
There are many heterogeneous catalysts and catalyst devices which are used to carry out a great number of chemical reactions. In the use of heterogeneous catalysis, the reaction rate for a catalyst device can be limited by the rate of bulk mass transfer of reactants to the catalyst surface or by the rate of pore diffusion of reactants within the pore structure of porous heterogeneous catalysts. Such limitations are widely described in the technical literature, for example in the books "The Role Of Diffusion In Catalysis", by C. N. Satterfield and T. K. Sherwood, published by Addison-Wesley Publishing Company (1963) and "Heterogeneous Catalysis In Practice", by C. N. Satterfield, published by McGraw-Hill Book Company (1980). The engineering design of heterogeneous catalytic reactors is frequently based on these limitations. New catalyst configurations which can reduce or perhaps even eliminate such mass transfer limitations would have substantial practical value.
In the field of catalytic filters, there are various devices which can simultaneously remove particulate matter and catalyze a reaction in the fluid being filtered. For example, G. F. Weber, et al., at the Energy and Environmental Research Center at the University of North Dakota have developed catalyst-coated fabrics suitable for filtration of coal combustion flue gas for particulate removal with simultaneous reduction of oxides of nitrogen contained in the flue gas ("Simultaneous NO.sub.x and Particulate Control Using A Catalyst-Coated Fabric Filter", American Society of Mechanical Engineers Paper 91-JPGC-FACT-2, presented at the International Power Generation Conference, Oct. 6-10, 1991, San Diego, Calif.). This device consists of fiberglass fabrics which are coated with vanadium oxide catalysts, which in the presence of added ammonia, efficiently remove particulates as a filter and reduce the oxides of nitrogen in the gas flowing through the fabric filter. This latter process is called selective catalytic reduction (SCR).
Similar fabric filter devices are disclosed by E. A. Pirsh U.S. Pat. Nos. 4,220,633 and 4,309,386.
One advantage of such catalyst coated filters is that two processes can be achieved in a single device. In the above instance, the processes are particulate removal and catalytic reduction of a gaseous contaminant.
A further advantage of such a catalytic filtration process is that particulate matter can be removed before the filtered fluid contacts the catalyst. In cases in which particulate matter poisons the catalyst, this prefiltration can prolong catalyst life. This can be important, for example, in SCR systems for reduction of oxides of nitrogen in combustion streams containing ash catalyst poisons.
Yet one further advantage of such a catalytic filter is that the reacting fluid flows through the catalyst coated filter in contrast to flowing over or around a catalyst coated support. For example, in a packed bed catalytic reactor, a reactant fluid flows through a packed bed of catalyst particles, and both bulk and pore diffusion limitations, as indicated above, can limit the reaction rate. A similar consideration holds for other catalytic reactor devices, such as monolith supported catalysts in which the reactant fluid flows through the passageways of a monolith support onto which a catalyst coating has been applied. For the catalytic filter, however, the reactant fluid flows through the pore structure of the catalyst, which is in fact the catalyst-coated filter. This flow configuration can greatly reduce or even eliminate bulk diffusion or pore diffusion limitations present in other more traditional catalytic reactors.
There are certain limitations of the catalytic fabric filters as embodied in the art described above. One is that the filter itself is not compact. Typical area/volume ratios for fabric bag filters are from four square feet per cubic foot (for a twelve inch diameter bag) to twelve square feet per cubic foot (for a four inch diameter bag). In contrast, the filter disclosed in U.S. Ser. No. 07/639,568 can have an area to volume ratio from about thirty square feet per cubic foot up to and greater than about one hundred seventy five square feet per cubic foot. Thus the filter of U.S. Ser. No. 07/639,568 has a compactness up to over forty-fold greater than that of typical fabric bag filters.
Another limitation of the prior art is that fabrics used in bag filters can have temperature limitations. Even filters produced from high temperature ceramic fibers can have such temperature limitations. In contrast the filter of U.S. Ser. No. 07/639,568 can be produced from high temperature ceramics. This can be an advantage when the reaction to be performed with the catalytic filter is at an elevated temperature.
Thus, the catalytic filter disclosed herein has the advantages of substantially eliminating diffusional limitations present in many heterogeneously catalyzed reactions, a very high compactness for a catalytic filter, and a capability for high temperature service.
The device of this invention has wide utility for filtration of gases and liquids while concurrently catalyzing a reaction within the filtered fluid. Applications of special importance are found in the field of air pollution control for combustion gases from which fly ash can be removed while simultaneously removing gaseous contaminants such as oxides of nitrogen, sulfur dioxide, and volatile organic vapors. Other applications for air pollution control and coal gasification exist in which it is desirable to remove particulate matter, followed by catalyzing a reaction of one or more gaseous species present. For example, oxidation processes to remove organic vapors from a variety of industrial sources may be used to remove a variety of air toxics enumerated in the 1990 Amendments to the Clean Air Act of 1970. The amendment identifies 189 air toxics, mainly organics, emitted by a variety of sources which must be controlled.